In 1998, the Illinois Department of Public Aid completed a study of payment accuracy, which they believe to be the first review of any state Medical Assistance Program. In their reviewed of a sample of 599 services, they interviewed clients and had medical experts review the claims. They found 95. 28% percent accuracy rate, plus or minus 2.31% of total dollars paid, which in 1998 would have been at least $113 million. The review also identified specific areas with much higher error rates, including non-emergency transportation, which had a 31% error rate.

The methodology used for this study serves as an example for other programs measuring payment accuracy or fraud.